The present invention relates to anodes for use in controlling corrosion and more particularly to providing a protective shield for such anodes so that the anode will perform its normal corrosion prevention function when immersed and will prevent accidental impact between the anode and other metallic objects or structures due to falls and/or accidents involving the anode.
Typically the corrosion of metals in sea water and other aqueous environments is an electrochemical process, wherein a flow of electrons takes place between certain areas of the metal surface in contact with the aqueous solution which is capable of conducting an electric current. The results of this electrochemical process is deterioration or eating away of the metal in those areas which are commonly referred to as anodes and at which the electrons leave the metal. Metal ions enter into the solution and the metal corrodes. The area receiving the electrons and becoming more negative is termed the cathode. There, electrons are discharged to the electrolyte. The cathode also must be present to complete a circuit. These two electrodes may comprise different metals or different areas on the same piece of metal due to impurities on the surface, differences in surface structure, etc. A common and well-known example of such an electrochemical action is the dry cell battery principle which provides energy to make the flashlight operate. Another similar example is an unprotected hull of the ship in sea water, the primary difference being that there is no switch to cut off the flow of electrons and, furthermore, the presence of the reaction is undesirable. In the case of a ship's hull, since the amount of electrons the earth can accept is near infinity, the process will continue until the hull is completely wasted away and as long as the flow of electrons is unimpeded through a path of low resistance the rate of wastage will be very rapid.
This electron flow is generally termed a galvanic process. If uncontrolled, it will tend to continue in the absence of the application of an equal or greater opposing force, which if applied, can greatly reduce the rate of the galvanic process or corrosion can be stopped completely. This is accomplished by supplying a sufficient number of electrons from another more powerful source so that the supply will make the structure to be protected the cathode. Corrosion, i.e. oxidation, does not occur at the cathode. The application of such an opposing force would provide the surplus of electrons without a loss of steel and is well known as cathodic protection. Using this type of protection means being prepared to sacrifice another material or energy for that purpose, which is the primary function of cathodic protection. By permitting the electrons from the galvanic anode to flow to a tanker hull or tank surface which has a more positive potential, i.e. cathodic, the desired protective function is obtained. The rate of electron flow depends on the driving force or potential difference between the metals, i.e. the voltage difference between the corroding site known as the anode and the protected site known as the cathode.
In the case of unprotected tanks, corrosion also takes place although there is no dissimilarity in metals. The corrosion is substantial because of the basic principle involved which is the same as explained above. Thus, surface imperfections, orientation of grains in a granular structure of the metal, lack of homogeneity, localized stresss and variations in environment cause the formation of large number of localized anodes and cathodes on the surface of the metal. The galvanic action results in anodic corrosion, i.e. corrosion at the anode site in comparison to the protected site which is the cathode. This corrosion is commonly prevented or at least minimized by using sacrificial anodes which are typically cast on support rods or cores and fixed at various locations through the tank in order to provide for complete protection of the tank. Hence, it is often desirable to place anodes at many elevations from top to bottom. Generally, the schemes employed for mounting the anodes are regulated by the U.S. Coast Guard. Presently various ship classification societies and the U.S. Coast Guard permit installation of aluminum anodes only in the lower levels of ballast tanks and other locations where a free fall of the anode cannot result in an impact of more than 200 foot pounds energy. Impacts of greater energy between hot aluminum and rusty steel can produce aluminum fragments capable of igniting petroleum gases. Present practice permits installation of the zinc anodes at greater heights because impact between zinc and steel are less hazardous.
It is apparent that in order to achieve a completely safe anode environment system, it is desirable to completely eliminate any metal-to-metal impact possibilities. The hazard of such impact is especially severe since there is a tendency to form a metallic smear on a target surface such as the steel structure which can result in a highly incendive thermit reaction. If these objectives to eliminate such impacts can be met, then the use of aluminum anodes will be safe at any elevation in the tank.
Various attempts at protection of anodes have been proffered in the prior art. These include those such as disclosed in U.S. Pat. No. 3,488,275, which provide protection from physical abuse during shipping in the form of a fabric container for the anode. U.S. Pat. No. 2,976,226 generally discloses providing a sleeve on an anode to protect against oil or reaction products but not impact; however, the anode is of the impressed current type variety. U.S. Pat. No. 3,196,101 provides a mesh wrapped about an impressed current anode for protecting it against any falling pieces, but does not suggest protecting against metal-to-metal impact. Other prior art such as U.S. Pat. No. 2,855,358 and 3,527,685 are considered deficient at least for the reason that they will not withstand the impacts for which the present invention was designed and still permit the anode to function normally when immersed in electrolyte.